1. Field of the Invention
The present invention relates to a method for injecting a sample being analyzed into the injection tube of a measurement cell, in particular of a densimeter.
2. Description of the Related Art
Of the physical measurements which have to be effected in the context of industrial processes, that of density figures among the most frequently necessary.
To that end, manufacturers market a range of densimeters based on various principles, all of which have advantages and disadvantages.
By way of example, a densimeter which can be used in a satisfactory manner to measure the density of a sample is equipped with a measurement cell comprising the following elements:                an isothermic enclosure which defines a measurement chamber at its inner portion;        a U-tube which is to be filled with the sample being analyzed and which extends inside the measurement chamber;        means for causing the U-tube to vibrate; and        means for reading the vibratory response of that tube.        
The U-tube of a densimeter cell of that type is secured to the measurement chamber at its free ends which project outwardly from that chamber in order to permit the injection of the sample being analyzed via an injection opening and its evacuation via an evacuation opening.
The principle of measuring the density of a sample by means of such a densimeter consists in causing the U-tube to vibrate at resonance frequency and in determining that frequency using the reading means.
The resonance frequency enables the density of the sample being analyzed to be calculated to a first approximation on the basis of a standard equation known per se and from a preliminary calibration of the densimeter.
The injection of a sample being analyzed into the U-tube of a densimeter cell of that type can be effected manually under pressure by means of a simple syringe equipped with an end-piece which is introduced into the injection opening.
However, such an operation is long and inconvenient and may also cause losses of sample.
In order to facilitate this injection, densimeters equipped with a device for the automatic injection under pressure of the samples being analyzed have already been proposed.
Those devices comprise a rotary dispensing carousel equipped at its periphery with a set of containers, in particular cylinders, for receiving samples being analyzed, which containers can be closed by a stopper.
The containers can be positioned successively in alignment with an injection station where they are coupled, on the one hand, to an injection duct connected to the injection opening of the densimeter cell and, on the other hand, to a duct connected to a pressure source.
The introduction of a sample being analyzed into a container provided on the dispensing carousel and the mounting on that container of the injection duct and the pressurising duct at the injection station are, however, long and tricky operations.
In parallel with those pressurised-injection devices, it has also already been proposed to inject the samples being analyzed into the densimeter cell under vacuum by connecting the evacuation opening of the U-tube to a vacuum source, in particular to a vacuum pump.
In such vacuum-injection devices, a container holding the sample being analyzed can be coupled to the injection opening of the U-tube via an injection duct.
Such devices have, however, the disadvantage of being suitable for analysing only low-viscosity samples and of requiring after each measurement a long and inconvenient rinsing step which additionally involves the use of a large amount of solvent, which is detrimental to the environment.
Also currently on the market are densimeters equipped with vacuum-injection devices which comprise an automatic dispensing carousel provided at its periphery with a set of containers for receiving samples being analyzed.
Such containers are positioned successively in alignment with an injection station where they are coupled to an injection duct connected to the injection opening of the densimeter cell.
Such devices having an automatic dispensing carousel nevertheless have the above-mentioned disadvantages of vacuum-injection devices as regards the impossibility of analysing viscous samples and the requirement to implement rinsing steps which necessitate large quantities of solvents.
In addition, the introduction of a sample being analyzed into a container provided on the dispensing carousel, and the mounting of the injection duct on that container at the injection station are again long and tricky operations.